The invention relates to a method for the injection of fuel into the combustion chamber of an internal combustion engine, especially for the injection of diesel fuel, and to an injection valve for performing the method.
DE 196 06 087 A1 describes a fuel injector, which has a plurality of primary injection ports and a plurality of secondary injection ports in an outwardly-opening valve tappet. The object of is fuel injector is to improve the efficiency of the fuel combustion when the engine is operating at high speed.
Similar injection valves are disclosed by DE 44 42 764 A1 and GB 2 113 303 A. DE 43 40 883 A1 likewise describes a fuel injector having an outwardly-opening valve tappet, with a valve head in which two or more parallel passages are arranged for the injection of fuel. The passages are separated from one another by a dividing wall and are successively opened during the opening stroke by a control edge on the valve head. The object of this arrangement is to maintain the spray direction in conventional diesel injection procedures, that is to say in the main combustion phase, irrespective of whether one or both passages are opened.
GB-A-1 521 065 describes a method for the injection of fuel into the combustion chamber of an internal combustion engine and an injection valve. The injection valve has two rows of injection ports, which are arranged in series and are designed to generate turbulence in the combustion chamber by rotation of the valve tappet in all load ranges.
Normally in a diesel engine, air is first compressed and then fuel is injected into the compressed, hot air where the fuel is instantly ignited. Additional fuel is then injected into the already burning fuel-air mixture. The mixture formation is essentially achieved in that the fuel jets injected under high impulse strike the wall of a recess formed in the piston and are there deflected, thereby creating a strong turbulence effect. In practice, very efficient mixing and hence mixture formation are thereby achieved during normal combustion.
This method functions relatively well at most operating points, especially in the operating mode for which the engine is optimally designed. It is virtually impossible to guarantee optimum mixing of fuel and oxygen, however, so that there is a risk that over-rich fuel zones are formed whereby increased amounts of soot are generated, or that virtually stoichiometrically mixed zones are formed in which very high combustion temperatures occur and thermal nitrogen oxide formation is increased.
It is the object of the present invention therefore to improve a method of the type as described in the introductory part in such a way that an improved mixture formation and hence good efficiency in all load ranges of the engine is achieved without increases in emissions. Another object of the invention is to create an injection valve for performing the method according to the invention.
In a method for the injection of Diesel fuel into a combustion chamber of a Diesel engine by way of a fuel injection valve with a plurality of pilot and main fuel injection ports, the fuel is injected during partial load operation only by way of the pilot injection ports such that a homogeneous mixture is formed and, under full load operation, fuel is additionally injected, preferably after a pause, through the main fuel injection ports in such a way that a fuel jet bundle is formed in combination with the fuel jets formed by the pilot ports which is extends over the whole periphery of the combustion chamber. The fuel injection valve has the main fuel ports arranged in a piezo-actuated tappet in series in the direction of the opening movement of the tappet behind the associated pilot ports.
With the method according to the invention, pre-homogenization and hence an extremely uniform fuel preparation are provided for already prior to the initiation of compression ignition. The pre-homogenization is achieved by injecting fuel through pilot injection ports to form pilot injection jets. The pilot injection jets are distributed around the periphery of the combustion chamber in such a way that thorough mixing of the fuel with the air present in the combustion chamber occurs at a point in time when the pressure and temperature of the compressed air have not yet reached a sufficiently high level for the onset of rapid ignition. This means that there is sufficient time for fuel atomization.
If the engine is operated at partial load, the injected fuel quantity, which, for this pre-homogenization may amount for example to as much as 50% of the total injection amount, is sufficient to achieve subsequent compression ignition with an engine power output sufficient for partial load operation.
The prior intensive mixing of air and fuel prevents soot formation during compression ignition, since there are no overly rich fuel zones present. The mixture preparation according to the invention also allows the mixture to be leaner, so that only minimal nitrogen oxide formation occurs. At the same time engine efficiency is improved and/or the fuel consumption is reduced.
However, the method according to the invention also functions under near full load operating conditions and during full load operation.
In this case for the pilot injection, which occurs in particular 100xc2x0 before the top dead center position of the associated piston to approximately 30xc2x0 before the top dead center positions, a uniform mixture preparation is provided. In order to then reach the upper partial load range or even full load range, the homogeneous fuel preparation according to the invention formed in the partial load range must be combined with the known conventional injection strategy, since, what matters here, is a high jet impulse, intensive jet-wall interaction and an optimum air utilization and turbulent mixing.
According to the invention, this is now achieved in that, under full load or nearly full load operating condition, the main jets discharge through the main injection ports distributed around the periphery are also activated. It is important here that the pilot injection jets and the main jets together produce common jet bundles, which with the injector fully opened behave like individual jets from large injection ports.
In a particular embodiment of the invention, this is achieved in that the pilot injection ports, which generate the pilot injection jets, are so arranged with regard to their orientations and the injection angle that the main jet emerging from a main injection port incorporates the respective associated pilot injection jets.
The inventors surprisingly found that, with a corresponding arrangement of the injection ports and coordination of the diameter ratios, wherein the main injection ports have distinctly larger diameters than the pilot injection ports, the main jets, owing to their higher impulse and the resulting higher penetration velocity, produce a suction effect, by means of which the respective pilot injection jets are sucked in and thereby combined with the respective main jet to form a common jet, resulting in uniform jet bundles distributed over the periphery.
The diameter ratios between main injection ports and pilot injection ports may be between 2:1 and 6:1. Other values are obviously still possible, and are within the scope of the invention.
When main injection is required for full load operation, one or more injection pauses are advantageously provided between the pilot fuel injection and the main fuel injection, which further enhance the homogenous mixing.
In a very advantageous embodiment, this method can be performed by an injection valve, which allows an outwardly-opening valve tappet to briefly perform a counter-movement during the injection phase, which briefly closes the pilot injection ports in order to provide for the desired injection pause. It has also been found that an injection valve, in which a piezo-ceramic actuating device is provided for the movement of the valve tappet, is particularly suitable herefor. In contrast to a pressure-controlled system, in which such precise positioning is not possible, a piezo-ceramic actuating device also allows intermediate positions to be assumed by the valve tappet.
The invention will become more readily apparent from the following description of an embodiment thereof, shown, by way of example only, in the accompanying drawings.